the best way to make water boil faster is to put on a tight fitting lid. Or move to a much higher altitude. which ever is easier.
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Sam HolderSep 3 '10 at 9:23

1

@Sam, I would avoid using altitude as a way to speed up boiling (not that it's even easy do). It boils faster because it's at a lower temperature, and for most uses (eg. Tea), the temperature of the water is an important factor, not just the fact that it's boiling.
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dsampleSep 26 '12 at 13:15

Actually adding salt to water makes it boil slower; it increases the boiling point so it takes a little longer to get there. It actually doesn't matter what you dissolve in water (or anything else). Adding a dissolved substance elevates the boiling point and lowers the freezing point.

I suspect it is because as the salt is dissolved, that portion of water (salty) is now denser and remains at the bottom of the pan and gets more exposure to the heated bottom. Pure speculation though.
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JaySep 3 '10 at 2:13

21

I would assume the instant boiling is an effect of adding nucleation sites, though this too is speculative.
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dmckeeSep 3 '10 at 2:17

5

@dmckee - I think that is right as well; it isn't that you've suddenly reached the boiling point, you just release a bunch of bubbles of oxygen that are trying to come out of solution.
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Michael at HerbivoraciousSep 3 '10 at 3:20

1

Try this in the microwave: Use a pyrex measuring cup, and nuke a cup of water for about 2 minutes. Watch during the last minute, so that you can time when it starts to boil. Lets assume 1:45. Now, do it again, but only nuke it for 1:40. Take it out, and add a spoonful of salt (Stand back!!!!) It should explode into a boil. What happened is you actually raised the temperature of the water above 100 deg. But the boiling action requires nucleation points (salt crystals!) In the pot on a stove, the bottom is above 100, but the top isn't. Adding salt causes the bottom to nucleate.
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Chris CudmoreSep 3 '10 at 15:03

4

Be very careful with the experiment that @chris suggests. Vibrations in the vessel can also nucleate the boiling, and people have been hurt when apparently quiescent liquids suddenly boil over. Ideally you would wear a full face shield, rubber apron and long rubber gloves. (I superheated half a cup of half-and-half and set it off in the middle of the kitchen, once. Splashes went five feet in every direction and the spousal unit and I both got multiple small burns.) That said, this is a fantastic demo.
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dmckeeOct 6 '10 at 19:05

Interesting point. But how much salt does one need to add to raise the temperature of boiling from 100 to 106 degrees? With sugar, you need 80% sugar concentration to reach 108° to 118° (the first stage given in candy charts).
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rumtscho♦Apr 17 '12 at 10:47

2) Solid NaCl actually releases heat when it is dissolved in water. Not very much heat, but if the water is on the narrow edge of boiling already, that added heat of dissolution can be enough to get things boiling a half a giffy sooner.

Again, these are both minor effects; the first merely looks a bit like near-boiling, the second probably can't be detected without a good stopwatch.

its because a lot of households have hard water which has many ions and a high boiling point. adding NaCl softens water and actually reduces ion content in the tap water making it easier to boil. everyone else is theoretically right to state that adding table salt to water increases boiling point but that is for pure water not tap water.

How does dissolving an ionic substance in it reduce ion content? Is it making something precipitate out?
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Peter TaylorJan 28 '12 at 10:18

Salt doesn't soften water. "Hard" water has calcium and magnesium compounds dissolved in it. They will still be dissolved in the water after adding salt, so adding the salt hasn't altered the hardness.
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David RicherbyAug 14 '14 at 18:28